1. Field
This invention relates generally to an electronically focusable lens and, more particularly, to an electronically focusable lens assembly including a Fresnel lens having a plurality of segments that are each curved in both the radial and rotational directions and a light filter positioned proximate to the Fresnel lens that has a display that is selectively controlled to make portions of the light filter transparent so that light only propagates through portions of each segment of the Fresnel lens that have the same focal length so as to instantaneously change the focal length of the lens assembly.
2. Discussion
The curvature of a traditional lens determines its focal length. Various optical systems, such as cameras, require a lens with an adjustable focal length. This generally requires a lens assembly that includes at least two lenses, where the lenses are movable relative to each other to change the focal length of the combination of the lenses. A typical lens assembly of this type is focused by mechanically moving the lens farther or closer in the direction normal to the lens plane. Further, the larger the desired aperture (diameter of the lens) of the optical system, the larger and heavier the lens assembly needs to be. As such, these types of optical systems generally are required to have moving parts, be relatively thick compared to the lens width, require time to move the lens in order to focus the lens assembly and have a relatively small aperture. Because of these disadvantages, these types of lenses are limited in their applications.
A Fresnel lens is one type of known lens that is generally more compact and lighter than a traditional lens. A typical Fresnel lens is divided into concentric annular segments, where each segment has a different angular orientation that defines stepwise discontinuities between the segments, and where the angular orientation of the lens segments increases and the thickness of the lens segments decreases towards the outer edge of the lens so that the light is diffracted properly for the lens focal length. This allows the overall thickness of the lens to be reduced compared to that of a traditional lens.
Electronically focusable lenses that do not require two lenses to be moved relative to each other are known in the art. In one particular design, the lens is made of a liquid crystal material, where different voltages applied to the lens cause the lens to change its curvature, which changes its focal length. When changing the focal length of this type of lens, the focal length of the lens must go through all of the focal lengths between the current focal length and the desired focal length, where it takes a certain amount of time for that to occur. In other liquid crystal lens designs, the lens is able to be electronically focused from one focal length to another focal length without going through all of the focal lengths in between. However, these types of lenses are often not scalable, require large voltages to operate and use exotic materials that are not readily available.